Processes for the thermal dimerization of conjugated diolefins are known. However, particularly in the thermal dimerization of butadiene, undesired high-molecular weight polymers are formed which are termed "popcorn" polymers due to their shape. So-called "germs" are formed which act as starting points for further polymerization. These "germs" primarily settle on the walls of the reactors and grow towards the interior of the apparatus, blocking the passageways and building up pressure drop across the apparatus.
Since the "popcorn" polymerizate is insoluble in common solvents it is impossible to wash it out. Owing to the strong adhesion and the partially tacky consistency, the mechanical removal is difficult, and it is impossible to remove it from the piping of commercial plants. Even if the apparatus were purged with utmost care, small particles remain which favor subsequent growth of undesired polymer. The "popcorn" polymerization thus causes loss of valuable raw material and noticeably increases the total production cost as a result of plugging of reaction apparatus and piping.
To avoid this undesired polymerization it has been suggested to use an organic solvent as reaction medium. In U.S. Pat. No. 2,411,822, for instance, kerosine is recommended as a solvent. In U.S. Pat. No. 3,454,665, a paraffinic washing oil with a boiling point of approx. 215.degree. C. is recommended. However, the amount of soluble polymerizate formed in this process is considerable, though "popcorn" polymerizate is not mentioned in these patent specifications.
According to Industrial and Engineering Chem., Vol. 39, No. 7, pages 830-837, gaseous nitrogen dioxide and aqueous sodium nitrite solutions are employed as general polymerization inhibitors. This publication made in 1947 refers to the prevention of styrene and butadiene polymerization in storage tanks and facilities connected in series with the styrene synthesis reactor. The aqueous solution of sodium nitrite is particularly referred to as an inhibitor for styrene polymerization and as an inorganic antioxidant. Its efficiency was tested in both cases at 55.degree. C. and it was found that (basis the inhibition effect at this temperature) styrene remained liquid for 100 to 200 days. At a temperature of 100.degree. C., which is a very high one for such facilities, styrene remained liquid for only 14 days. The use of aqueous sodium nitrite solution in polymerization processes as such for preventing formation of "popcorn" during polymerization is not taught in this publication. This understandable, because the polymerization processes as such, particularly the thermal polymerization processes are performed at noticeably higher temperatures.
For preventing "popcorn" polymerization in the thermal dimerization of conjugated olefins a great variety of inhibitors has been suggested. In U.S. Pat. No. 2,943,117 an aromatic solvent, e.g. benzene in combination with an aqueous solution of a diamine or with aqueous ammonia solution of a diamine or with aqueous ammonia solution is recommended. However, using this process the formation of high-molecular polymers cannot be completely prevented.
In the German Auslegeschriften Nos. 20 38 311 and 21 15 858 the use of N-substituted N-nitrosohydroxylamines is suggested. However, these systems are expensive and can only be used at temperatures of up to 140.degree. C.
According to DE-OS 18 16 826 dating from 1968, the susceptibility of butadiene to polymerize in solutions is lowered by adding aromatic nitro-compounds as polymerization inhibitors. On page 2, lower paragraph of said publication it is literally stated that at room temperature or lower temperatures the polymerization of butadiene can be prevented to a certain extent by adding conventional, known polymerization inhibitors, e.g. hydroquinone, 4-tert. butyl catechol, betanaphthyl amine, methylene blue, sodium nitrite etc.:
"However, the aforementioned polymerization inhibitors are not satisfactorily efficent with respect to preventing polymerization of butadiene, if the butadiene-containing solution is exposed to relatively high temperatures, e.g. 80.degree. C. to 160.degree. C. or even higher for an extended period of time."
Hence, it may be understood from said Offenlegungsschrift that sodium nitrite is unsuitable for inhibiting "popcorn" formation at the temperatures stated therein.